Almost half of NI's schools are in budget deficit, according to new figures obtained by BBC News NI.
The Education Authority (EA) has analysed the financial position of about 1,000 schools for 2018-19.
Its figures show that 446 schools are projected to be in the red in 2018. Of that number, 352 have seen their deficits increase since 2017-18.
All schools are expected to submit their financial plans for each school year to the EA.
The budget pressures faced by schools are "quite intense", according to Northern Ireland's most senior civil servant.
David Sterling said the Department of Education was "doing all it can" to make sure "our schools are suitably and sufficiently funded".
But he admitted it was "difficult" and said there needed to be "long-term planning" and "major transformation" for public services.
'Support at-risk schools'
In 2018, for the first time, the EA has put schools into a number of categories depending on their budgetary situation.
In the most serious category, 97 schools have increasing deficits of more than 5% of their total yearly budget and do not meet key sustainability criteria.
A total of 130 schools have increasing deficits of more than 5% of their total yearly budget but are judged to be sustainable.
As a result, the EA is to appoint specialist staff to work with each of those 227 schools on their budget.
The EA said the category given to a school "is not a judgement on their financial management or stewardship".
"The categorisation is to enable EA to target its support to those schools at most financial risk," it added.
The EA had previously warned that schools in Northern Ireland were set to overspend their budgets by about £33m in 2018-19.
The Northern Ireland Audit Office has said that school budgets have reduced by 10% in real terms over the past five years.
'Beyond a joke'
A number of school principals have said they cannot afford to send teachers to training that would enable them to help pupils with speech and language problems.
The EA's language and communication service has organised more than 30 training courses for teachers in the next five months but cannot pay schools for the cost of substitute teacher cover to enable staff to attend.
The principals of Dromara Primary School and Comber Primary School in County Down and Maghaberry Primary School in County Armagh are among those who cannot send their teachers to the training.
They estimate that it would cost about £200 to send a teacher to one session of the training.
Andy Armstrong, the principal of Dromara Primary School, said the situation was "beyond a joke".
"I just felt so deflated that while courses are being offered we can't afford to send anyone," he said.
A spokesperson from the EA acknowledged that schools faced "unprecedented pressures".
"Many school leaders have told us of the intolerable strain that the deteriorating financial position has placed upon them," they said.
"EA will continue to advocate for additional funding for schools and services to support schools, children and young people."
'Spiralling out of control'
Deidre Gillespie, principal of St Mary's Grammar in Magherafelt, County Londonderry, said there needs to be a "root and branch review" of how money is spent in the education system.
"Over the past six to seven years, schools have embarked on a series of cost-cutting measures and we've got to the stage now where there are no longer any savings to make within our budgets," she said.
"Ninety percent of our costs go to staffing, which increase every year and upon which we have very little control.
"It means that last 10% to 15% of our budget, where we've made those cost-saving measures over a number of years, we're really not able to create any more cost-saving.
"We're in a crisis situation and we're getting to the point very, very soon where the system is spiralling out of control."
Comber Primary School principal Chris Logan said that the funding relied on by the EA was being pared away.
"Our school, like many others, would also be unable to attend [training] due to lack of funding and the need to closely monitor and limit expenditure due to our deficit position," he said.
"I look forward to the time when we are able again to access incredible training opportunities and not turn them down due to lack of funding."
Graham Gault, of Maghaberry Primary School, had previously told a Westminster Committee that parents were donating toilet roll to his school due to budget cuts.